The oil tanker Prestige, which split in two and sank off the coast of Spain on Tuesday, was an ageing single-hulled vessel.

Such ships are vulnerable to oil spills after collisions or grounding. Serious questions have also been raised over the seaworthiness of the 26-year old Prestige.

The vessel was one of many tankers built quickly and cheaply in Japan during the country's economic boom in the late 1970s.

Major repairs were ordered to the ship in 2001 after American Bureau of Shipping inspectors found cracks in sections of its hull, it has emerged.

Welding to these cracks carried out in China may have split causing the Prestige to sink, some experts have speculated.

The ship, managed by Athens based company Universe Maritime, was bound from Latvia to Singapore when it got into difficulties during a storm in the Bay of Biscay last week.

The vessel was carrying 77,127 metric tons of fuel oil.

International organisations are working to phase out single hulled tankers over the next decade, but large numbers are still in service.

Under EU legislation drawn up after a 1999 oil spill, tankers more than 25-years old will not be allowed to trade in Europe after 2005.

In April 2001 the International Maritime Organisation (IMO) decided single-hulled tankers built in 1973 or earlier should be withdrawn by 2007, and more recent ones by 2015.

However, Lloyds Marine Intelligence estimate that 52% of tankers of more than 10,000 tons currently in operation are single-hulled.

James Wilkes, a spokesman for Maritime consultancy MRC, told BBC News Online that varying regulations and levels of implementation creates the opportunity for ships to slip through the net.

Rules governing single-hulled ships vary between those instituted by the IMO, those of organisations like the EU, and the laws of the flag state authorities.

Confusion

There is also confusion about defining double-hulled tankers.

Some vessels have only partial double-hulls - with the sides of the ships having two hulls, but only a single bottom. These are not included as double-hulled in official statistics.

The Prestige was built in 1976

According to news agencies, the Prestige was last inspected in 1999. Universe Maritime, which manages the vessel, say that the ship's certificates were checked in St Petersburg in October and found to be in order.

The Spanish Government has said that the ship should have been inspected again at her most recent call at Gibraltar.

But MRC spokesman James Wilkes told BBC News Online that this call was just a routine "bunker call" which does not involve docking, and inspections are not normally carried out during such stops.